The invention relates to a method for controlling a connection, especially a wireless connection, between two modems in the event of disturbance, this method comprising initiating a so-called retrain procedure in the event of disturbance when either one of the modems receives a signal of unsatisfactory quality, in which retrain procedure the modems transmit predetermined retrain signals to each other in order to achieve synchronism between them. The term wireless connection is used herein to refer to a connection implemented in such a manner that it is at least partly wireless.
A wireless radio network is originally intended for transmission of speech but it is also suitable for transmission of data. Data transmission in a radio network is carried out by means of a modem in a terminal and a mobile phone, e.g. a cellular phone. It is possible to use, for example, a modem positioned in a portable computer or a modem designed for a conventional telephone network. The modem is connected, for example, to a cellular phone by a separate adapter, which, for instance, adjusts the transmission level.
FIG. 1 of the attached drawings is a block diagram of equipment, known per se, for transmitting data between a computer 11 (typically a portable computer) or a terminal with a similar function and another terminal (not shown) connected to it through a radio network RN. The equipment comprises a modem 12, which is connected to the computer 11 and, via the above-mentioned adaptor 19, to a mobile phone 13. The modem 12 comprises an interface unit 14 (e.g. conventional RS-232 serial interface), to which the terminal 11 is connected, an error correction unit 16 connected to the interface unit, a data pump 17 connected to the error correction unit, a line interface unit 18 connected to the data pump 17, and a control processor 15 for controlling the operation of the modem. The actual modem part consists of the data pump 17, which modulates digital signals supplied from the terminal 11 to a form suitable for an analog telephone network and demodulates signals supplied from the telephone network to a form suitable for the terminal equipment. The data pump 17 is connected to the telephone network (adapter) through the analog line interface unit 18. The control processor 15 of the modem controls the data pump and performs error correction and all other functions of the modem, such as communication with the computer through the interface unit 14. The error correction unit 16 receives data transmitted through the interface unit, packs the data in frames of a fixed size, and transmits the frame through the data pump. At the reception end, the data pump 17 converts the received analog signal into digital form, whereafter the receiving error correction unit examines the received frame and detects any possible errors, which are corrected by means of retransmission.
In view of data transmission in a mobile telephone network, the most significant component of the modem is the data pump 17. The CCITT (at present the ITU-T) has defined a number of modulation standards of different rates for use in a general switched telephone network. These standards are illustrated in the following table.
______________________________________ Modula- Carrier Symbol Recom- Rate tion frequencies rate mendation (bit/s) method (Hz) (1/s) ______________________________________ V.21 0-300 FSK 1080/1750 300 V.23 1200/75 FSK 1700/420 1200/75 V.22 600 QAM 1200/2400 600 1200 V.22 bis 1200 QAM 1200/2400 600 2400 V.32 4800 QAM and 1800 2400 9600 TCM V.32 bis 4800 QAM and 1800 2400 7200 TCM 9600 12000 14400 ______________________________________
The abbreviations used in the third column of the table above have the following meanings: FSK=Frequency Shift Keying, QAM=Quadrature Amplitude Modulation, and TCM=Trellis Code Modulation.
As appears from the table, the modems according to recommendation V.32 bis offer a high rate and are thus a reasonable option even in radio networks. A V.32 bis connection is established between modems by a start-up procedure described on page 10 of recommendation V.32 bis. There are different procedures for the calling and the answering channel (modem). The establishment of the connection is described more closely in the above-mentioned recommendation, pages 14 to 16, which are referred to for a more detailed description. In practice, the start-up and the retrain procedure are almost identical, as the retrain procedure forms the synchronization part of the start-up procedure.
If the modems lose synchronism during the connection, the so-called retrain procedure, i.e. resynchronization of the modems, is carried out. The operating principle of the retrain procedure is illustrated in the figure on page 11 of the above-mentioned recommendation. A more detailed description of the retrain procedure is given on page 16 of the above-mentioned recommendation.
In a radio network there are often situations where modems lose synchronism, and the above-mentioned retrain procedure must be carried out. Such situations may occur in a wireless network, such as a cellular network, for the following reasons:
Synchronism is lost on account of change of base stations. PA1 Noise is suddenly present on the connection, which prevents data from being transmitted at the current rate. PA1 A cellular connection from a moving car brings about changes in the field, and radio waves reflected from buildings cause interference. PA1 The quality of the line is low. (Operations in cellular networks are always performed within the area of a certain base station. The quality of the line is high near the base station but gets lower as the distance from the base station grows. In the area where base stations are changed the quality is usually low.) PA1 The internal signalling (change of base stations, transmission level, field strength) carried out on a speech channel of a cellular network causes breaks in data transmission. Depending on the duration and frequency of the signalling, unnecessary resynchronizations, which weaken the performance, may take place on the data connection. PA1 With a moving car, there are always shadow regions, in which the connection does not work in practice. The connection is, however, restored when the shadow region is left behind. A normal data connection is disconnected at such locations unless) special measures are taken to prevent it. A shadow region may also occur, for instance, at traffic lights, in which case the car may remain in the shadow region for rather a long time, even dozens of seconds. PA1 The (wireless) network disconnects the connection.
In a normal telephone network the retrain procedure usually succeeds, and the modems return to the data transmission state. In a radio network, however, the quality of the connection varies, which causes problems. Modems designed for use in a normal telephone network usually disconnect the connection after two or three retrain attempts. In a mobile telephone network it is essential that the connection is not disconnected on account of disturbances in the network. In a normal case, the connection is disconnected by means of LINK DISCONNECT messages from a V.42 or an MNP error corrector or a GSTN CLEARDOWN sequence defined in recommendations V.32 and V.32 bis. If the (wireless) network disconnects the connection, the modems proceed to the retrain state and after a given time disconnect the connection as the retrain procedure does not succeed (the far end is no longer on the line). The given time must be extended even to several minutes so as to prevent unnecessary disconnection of the connection.
In a mobile telephone network, the change of base stations or the shadow regions due to the environment may thus lead to loss of synchronism between data pumps, wherefore the data pumps start the retrain procedure in order to be able to return to the data transmission state. Disturbances in the network may, however, lead to failure of the retrain procedure, in which case the procedure is started over again. When the quality of the connection is improved, the retrain procedure usually succeeds, and the modems return to the data transmission state. Very often, however, the retrain procedure does not succeed for some unaccountable reason no matter how high the quality of the mobile telephone connection is. In that event, there is no other alternative than to disconnect the connection and to establish it all over again.
As appears from the above, the problems caused in data transmission by wireless connections are such that the current methods do not provide a reliable and sufficiently rapid way to maintain or disconnect a connection.